1. Field of the Invention
The present invention relates generally to the field of disc drive storage, and more particularly to multilayer magnetic media used in data storage devices.
2. Description of the Related Art
Conventional disc drives are used to magnetically record, store and retrieve digital data. Data is recorded to and retrieved from one or more discs that are rotated at more than one thousand revolutions per minute (rpm) by a motor. The data is recorded and retrieved from the discs by an array of vertically aligned read/write head assemblies, which are controllably moved from data track to data track by an actuator assembly.
The three major components making up a conventional hard disc drive are magnetic media, read/write head assemblies and motors. Magnetic media, which is used as a medium to magnetically store digital data, typically includes a layered structure, of which at least one of the layers is made of a magnetic material, such as CoCrPtB, having high coercivity and high remnant moment. The read/write head assemblies typically include a read sensor and a writing coil carried on an air bearing slider attached to an actuator. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. The actuator is used to move the heads from track to track and is of the type usually referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing closely adjacent to the outer diameter of the discs. Motors, which are used to spin the magnetic media at rates of higher than 1,000 revolutions per minute (rpm), typically include brushless direct current (DC) motors. The structure of disc drives is well known.
Magnetic media can be locally magnetized by a read/write head, which creates a highly concentrated magnetic field that alternates direction based upon bits of the information being stored. The highly concentrated localized magnetic field produced by the read/write head magnetizes the grains of the magnetic media at that location, provided the magnetic field is greater than the coercivity of the magnetic media. The grains retain a remnant magnetization after the magnetic field is removed, which points in the same direction of the magnetic field. A read/write head that produces an electrical response to a magnetic signal can then read the magnetization of the magnetic media.
Magnetic media structures are typically made to include a series of thin films deposited on top of aluminum substrates, ceramic substrates or glass substrates. FIG. 1 illustrates a conventional magnetic media structure having a substrate 105 made of aluminum with nickel phosphorous (NiP) coating, a first layer 110 made of chromium (Cr), a second layer 120 made of chromium-tungsten (CrW), chromium-molybdenum (CrMo), or chromium-titanium (CrTi), a third layer 130 made of cobalt-chromium (CoCr), cobalt-chromium-tantalum (CoCrTa), or cobalt-chromium-ruthenium (CoCrRu), a fourth layer 140 made of cobalt-chromium-platinum-boron (CoCrPtBo), a fifth layer 150 made of a material having a high magnetic saturation Ms alloy, and a sixth layer 160 which acts as an overcoat and is typically made of a carbon containing material.
The magnetic media structure described with reference to FIG. 1 above is made using conventional magnetic media manufacturing processes. Conventional media manufacturing processes typically start by cleaning the substrate 105 and then texturing the substrate. Typically the substrate 105 is made of aluminum with a nickel phosphorous coating that acts as a hard surface that can be textured. After the substrate has been textured it is usually cleaned again before the subsequent layers are deposited on top. The deposition process includes sputtering target material of usually the same material as their respective layers so that thin films of the sputtered material grow on the substrate. The deposition process is usually done either at ambient temperatures or at elevated temperatures and only after the deposition chamber has been evacuated to low pressures. Multilayer longitudinal media is typically deposited at elevated temperatures in order to achieve desirable magnetic properties.
The magnetic layers of the structure can be used for perpendicular or longitudinal recording media, which include a single or a couple of magnetic layers wherein the thickness of each layer can range from about 10 Å to about several hundred angstroms, are typically deposited onto substrates that have been heated to high temperatures, such as 250° C. Growing thin films on hot substrates reduces noise by promoting desired crystallographic orientations and by enhancing Cr segregation into grain boundaries. During deposition, the higher substrate temperature enhances molecule mobility permitting desired crystallographic orientations to grow and enhancing Cr segregation into grain boundaries reduces exchange coupling of the grains reducing noise.
In addition to using temperature to make magnetic alloys with better crystallographic orientation or high anisotropy, the reduction of magnetic film thickness is also used. This is usually accomplished by using alloys having high magnetic remnant moment (MrT), high orientation ratio (OR) and high magnetic saturation moment (Ms). Additionally, using alloys having uniform and small grain size help make media with such performance.
The problems with some of these techniques are that magnetic media having high coercivity (Hc) and high anisotropy is limited head writability. The increase of Hc from larger grain size also negatively affects the media signal to noise ratio. Although higher MrT orientation ratio can be achieved through texture and cleaning processes, the improvements are limited. Another approach for film thickness reduction is using high Ms alloy through reducing Cr atomic percentage. However, high Ms alloys with lower Cr at % usually has significant impacts on the grain segregation, which negatively affects the media by increasing the media noise. Although continuous grain size reduction improves signal to noise properties and is desirable, it is limited by media thermal decay due to superparamagnetic behavior. The nominal grain size for longitudinal media has been reduced to 10 angstroms or less with a grain distribution of 2-3 angstroms of standard deviation.
Materials having four elements CoCrPtB, as discussed with reference to FIG. 1 have been commonly used for magnetic layers of magnetic media. These materials have been optimized to have high Cr and enough B to improve the magnetic media. However, there is a limitation to the amount of optimization of these compositions that can be done because the Cr percentage has increased to the point that the materials are on the verge of being non-magnetic. Increasing the Cr and B percentage in these materials decreases the coercivity Hc and remnant moment Mrt of the magnetic media to the point that it is unusable.
Despite the improvements that have been made in reducing noise of recording media having high areal density there are still many problems that need to be addressed including making desirable thermally stable magnetic media with higher signal to noise ratios and narrow track recording widths. Therefore what is needed is a system and method that results in a thermally stable magnetic media for recording information that has high areal density and reduced noise.